Make compatible with freezed package

[mkranus]

Freezed package requires the JsonKey annotation to be inserted into the constructor, which is then copied to the generated classes. But currently this use generates a warning, and this PR adds an additional TargetKind so the warning is no longer generated.

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[mkranus]

@googlebot I signed it!

[vi-k]

@JsonSerializable
factory ...

-> The annotation 'JsonSerializable' can only be used on classes.

No TargetKind for factory.

[vi-k]

See rrousselGit/freezed#474
and
dart-lang/sdk#46844

[princeteck]

Any update on this one @mkranus